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بسم هللا الرحمن الرحيم ً ّ ً ”ألم ترأن هللا أنزل من السماء ماء فأخرجنا به ثمرات مختلفا ٌ ُ َ ُ ٌ ْ ٌ ألوانها ومن الجبال جدد بيض وحمرمختلف ألوانها وغرابيب سود“ صدق هللا العظيم (سورة فاطر -أية رقم )27 Optical mineralogy Is the science which show the behavior of the mineral react with light What is the optical mineralogy? All the crystalline materials react with light in different way The interaction between light and crystalline materials depend on the internal structure of materials Most of materials are react with light by different way • • • • • المعادن الشفافة Transparent التى تنقل الضوء من خاللها وتدرس بواسطة الميكرسكوب المستقطب وتعتمد على اعداد شريحة رقيقة سمكها اليتعدي مم.03 المعادن النصف شفافة translucentهى التى تسمح لجزء يسير من الضوء الساقط عليها بالنفاذ وتمتص أو تعكس الجزء الباقى المعادن المعتمة opaque minerals تدرس بواسطة الميكرسكوب العاكس وتعتمد على اعداد سطح مصقول تماما يتم صقله بواسطة وسائل خاصة Light Light- a form of energy, detectable with the eye, which can be transmitted from one place to another at finite velocity(300,000ث/(كم. White or visible light(3900-7700), that which the eye detects, is only a fraction of the complete spectrum Ray path: part of light following wavepath Beam: is the many of parallel ray Wavelength: it is the distance between similar point (crest to crest)( trough to trough) Amplitude: the distance between crest to path ray A2 α T Symmetric and asymmetric waves symmetric waves: amplitude line divided waves in to the same part and vibration direction perpendicular to ray path Asymmetric waves: amplitude line divided waves in to different part and vibration direction not perpendicular to ray path Types of light Monochromatic light: light which formed from one wavelength Polychromatic light: light which composed of ray have different wave length Polarization of light When light ray vibrate in one direction All of this introductory material on light and its behaviour brings us to the most critical aspect of optical mineralogy - that of Polarization of Light. Light emanating from some source, sun, or a light bulb, vibrates in all directions at right angles to the direction of propagation and is unpolarized. In optical mineralogy we need to produce light which vibrates in a single direction and we need to know the vibration direction of the light ray. These two requirements can be easily met but polarizing the light coming from the light source, by means of a polarizing filter. Minerals can be subdivided, based on the interaction of the light ray travelling through the mineral and the nature of the chemical bonds holding the mineral together, into two classes: Isotropic Minerals Isotropic materials show the same velocity of light in all directions because the chemical bonds holding the minerals together are the same in all directions, so light travels at the same velocity in all directions. Examples of isotropic material are volcanic glass and isometric minerals (cubic) Fluorite, Garnet, Halite In isotropic materials the Wave Normal and Light Ray are parallel. Anisotropic Minerals Anisotropic minerals have a different velocity for light, depending on the direction the light is travelling through the mineral. The chemical bonds holding the mineral together will differ depending on the direction the light ray travels through the mineral. Anisotropic minerals belong to tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems. In anisotropic minerals the Wave Normal and Light Ray are not parallel PHASE AND INTERFERENCE RETARDATION - (delta) represents the distance that one ray lags behind another. Retardation is measured in nanometres, 1nm = 10-7cm, or the number of wavelengths by which a wave lags behind another light wave. Before going on to eamine how light inteacts with minerals we must define one term: The relationship between rays travelling along the same path and the interference between the rays is illustrated in the following three figures. 1.If retardation is a whole number (i.e., 0, 1, 2, 3, etc.) of wavelengths. The two waves, A and B, are IN PHASE, and they constructively interfere with each other. The resultant wave (R) is the sum of wave A and B. 1.When retardation is = ½, 1½, 2½ . . . wavelengths. The two waves are OUT OF PHASE they destructively interfere, cancelling each other out, producing the resultant wave (R), which has no amplitude or wavelength. If the retardation is an intermediate value, the the two waves will: be partially in phase, with the interference being partially constructive be partially out of phase, partially destructive. Refractive index Index of Refraction in Vacuum = 1 and for all other materials n > 1.0. Most minerals have n values in the range 1.4 to 2.0. A high Refractive Index indicates a low velocity for light travelling through that particular medium. Snell's Law Snell's law can be used to calculate how much the light will bend on travelling into the new medium. If the interface between the two materials represents the boundary between air (n ~ 1) and water (n = 1.33) and if angle of incidence = 45°, using Snell's Law the angle of refraction = 32°. The equation holds whether light travels from air to water, or water to air. In general, the light is refracted towards the normal to he boundary on entering the material with a higher refractive index and is refracted away from the normal on entering the material with lower refractive index. In labs, you will be examining refraction and actually determine the refractive index of various materials. Critical angle and total reflection POLARIZATION OF LIGHT 1. Reflection Unpolarized light strikes a smooth surface, such as a pane of glass, tabletop, and the reflected light is polarized such that its vibration direction is parallel to the reflecting surface. The reflected light is completely polarized only when the angle between the reflected and the refracted ray = 90°. ميكن ن ن ن ن ن ن ن ن ننملستقط ن ن ن ن ن ن ن ن ن ن س ن ن ن ن ن ن ن ن ن ن س ن ن ن ن ن ن ن ن ن ن س طن ن ن ن ن ن ن ن ن ن ق اىل: -1أل س كان ك ة :ذرعسوقاعدةس سو نبوبةس سو طن س لن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن نندو ر -2أل ن ن ن س لب ن ن ن ة :لع ن ن نناشسو لتن ن ن ة اشسوسعد ن ننةسبس ت ن ن نندس ومكثفن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن نناشس ل ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ننو -3ن ن ن ن ن ن ن ن ن ن ن س أل ن ن ن ن ن ن ن ن ن ن ن ق ا س :طن ن ن ن ن ن ن ن ن ن ن ق سو ن ن ن ن ن ن ن ن ن ن ن -4ن ن س ة ننا ة :نندرس ل ننو س( من ن ةس ن ن ) ،سحا ن ن س إ ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن سل ن ن ن ن ن ن ن ن ن ن ن ن ن ن ننو سوم ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن اشس ل ن ن ن ن ن ن ن ن ن ن ن ن ن ن ننو -5لت ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن س أل ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ن ننا ة -1األجزاء امليكانيكية: أ -أنبوبة اجملهرmicroscope tube : هى اجلزأ العلوي املتحرك من اجملهر ىف طرفه العلوي تثبت العينيات وىف اجلزأالسفلى تثبت الشيئيات وحتت العينيات توجد عدسة برتراند وحتتها يوجد احمللل مث فتحة الشرائح األضافية( )gypsum plate, quartz wedge, mica plate ب -ذراع وقاعدة المجهر Arm and base لتثبيت والحمل وتتصل بهما األجزاء المختلفة ووسيلة ضبط الصورة تكون مثبتة فى ذراع المجهر ( )coarse and fine adjustmentوذلك برفع وخفض األنبوبة أما فى األنواع الحديثة فتضبط الصورة برفع وخفض المسرح بواسطة الضابط الخشن والناعم اللذان يكونان على نفس المحور ج -مسرح المجهرmicrscope stage: مسرح دائري مدرج كل 10درجات من صفر الى 360ويقاس مقدار دورانه بالنسبة لمقياس ورنيه وتوجد خمس فتحات ثالثة لتثبيت المسرح الميكانيكى او المسرح الجامع على سطح المسرح الدوار واما الفتحتان األخريتان فتستخدمان لتثبيت ماسكين لتثبيت القطاع على المسرح ويوجد مسمار عند حافة المسرح لتثبيت المسرح وعدم السماح له بالدوران د -المسرح الميكانيكى mechanical stage وذلك لتحريك الشريحة فى اتجاهين متعامدين Isotropic minerals Isotropic materials show the same velocity of light in all directions because the chemical bonds holding the minerals together are the same in all directions, so light travels at the same velocity in all directions. Examples of isotropic material are volcanic glass and isometric minerals (cubic) Fluorite, Garnet, Halite In isotropic materials the Wave Normal and Light Ray are parallel. category Formula (repeating unit) Strunz classification Crystal symmetry Unit cell Identification Color Crystal habit Crystal system Cleavage Fracture Tenacity Mohs scale hardness Luster Streak Diaphaneity Specific gravity Optical properties Refractive index Fusibility Solubility Halide mineral CaF2 03.AB.25 Isometric H–M Symbol 4/m 3 2/m a = 5.4626 Å; Z=4 Colorless; although samples are often deeply colored owing to impurities. Well-formed coarse sized crystals; also nodular, botryoidal, rarely columnar or fibrous; granular, massive Isometric, cF12, SpaceGroup Fm3m, No. 225 Octahedral, perfect on {111}, parting on {011} Subconchoidal to uneven Brittle 4 (defining mineral) Vitreous White Transparent to translucent 3.175–3.184; to 3.56 if high in rare-earth elements Isotropic; weak anomalous anisotropism 1.448–1.433 3 slightly water soluble and in hot HCl acid CaF2 الفلوريت RELIEF - the degree to which a mineral grain or grains appear to stand out from the mounting material, whether it is an immersion oil, Canada balsam or another mineral. Refractometry involves the determination of the refractive index of minerals, using the immersion method. This method relys on having immersion oils of known refractive index and comparing the unknown mineral to the oil. BECKE LINE - a band or rim of light visible along the grain boundary in plane light when the grain mount is slightly out of focus. To observe the Becke line: use medium or high power, close aperture diagram, for high power flip auxiliary condenser into place. Microscopic properties to isotopic minerals 1-Crystal Habit ( prismatic, fibers, foliated, circular) 2-Crystal form ( euhedral-subhedral- anhedral) 3-Colors( green , yellow, colorless….. 4-Cleavage ( weak plane in minerals) 5- Inclusions (solid, fluid, gas) 6- alteration( garnt- chlorite) Selective Absorption This method is used to produce plane polarized light in microscopes, using polarized filters. Double Refraction INTERFERENCE PHENOMENA The colours for an anisotropic mineral observed in thin section, between crossed polars are called interference colours and are produced as a consequence of splitting the light into two rays on passing through the mineral. RETARDATION Monochromatic ray, of plane polarized light, upon entering an anisotropic mineral is split into two rays, the FAST and SLOW rays, which vibrate at right angles to each other. Due to differences in velocity the slow ray lags behind the fast ray, and the distance represented by this lagging after both rays have exited the crystal is the retardation - D. MONOCHROMATIC LIGHT POLYCHROMATIC LIGHT Michel Levy Chart EXTINCTION Anisotropic minerals go extinct between crossed polars every 90° of rotation. Extinction occurs when one vibration direction of a mineral is parallel with the lower polarizer. As a result no component of the incident light can be resolved into the vibration direction of the upper polarizer, so all the light which passes through the mineral is absorbed at the upper polarizer, and the mineral is black. Upon rotating the stage to the 45° position, a maximum component of both the slow and fast ray is available to be resolved into the vibration direction of the upper polarizer. Allowing a maximum amount of light to pass and the mineral appears brightest. Types of Extinction 1. Parallel Extinction The mineral grain is extinct when the cleavage or length is aligned with one of the crosshairs. The extinction angle (EA) = 0° Orthopyroxene, biotite 2. Inclined Extinction The mineral is extinct when the cleavage is at an angle to the crosshairs. EA > 0° clinopyroxene, hornblende 3. Symmetrical Extinction The mineral grain displays two cleavages or two distinct crystal faces. It is possible to measure two extinction angles between each cleavage or face and the vibration directions. If the two angles are equal then Symmetrical extinction exists. EA1 = EA2 Amphibole, calcite 4. No Cleavage Minerals which are not elongated or do not exhibit a prominent cleavage will still go extinct every 90° of rotation, but there is no cleavage or elongation direction from which to measure the extinction angle. e.g. Quartz, olivine ACCESSORY PLATES The accessory plates allow for the determination of the FAST (low n) and SLOW (high n) rays which exit from the mineral being examined. Accessory plates * All accessory plates used are constructed such that the slow vibration direction is across the width of the plate, the fast vibrations direction is parallel to the length. * Accessory Plates are inserted into the microscope between • the objective lens and the upper polar, in the 45° position. Gypsum plate (first order Red color)• Mica plate (first order white color)• Quartz wedge (variable retardations)• To Determine the Vibration Direction in a Mineral. If the colour increased, went up the chart, then the slow ray in the accessory plate is parallel to the slow ray in the mineral grain. If the colour decreased, went down the chart, then the slow ray of the accessory plate is parallel with the fast ray of the grain. SIGN OF ELONGATION Length fast means that the fast ray of the mineral vibrates parallel with the length of the elongate mineral or parallel to the singel cleavage, if present. This is also referred to as NEGATIVE ELONGATION, as the overall total retardation is less than that exhibited by the mineral prior to the accessory plate being inserted. Length slow means that the slow ray of the mineral vibrates parallel with the length of the mineral or the single cleavage, if present – POSITIVE ELONGATION, the total overall retardation is greater than that exhibited prior to the accessory plate being inserted. Only minerals which have an elongate habit exhibit a sign of elongation. Relief Minerals which display moderate to strong birefringence may display a change in relief as the stage is rotated, in plane light. This change in relief results from the two rays which exit the mineral having widely differing refractive indices. Pleochroism With the upper polar removed, many coloured anisotropic minerals display a change in colour - this is pleochroism or diachroism. Produced because the two rays of light are absorbed differently as they pass through the coloured mineral and therefore the mineral displays different colours. Pleochroism is not related to the interference colours Uniaxial Minerals Uniaxial minerals have only one optic axis, and belong to the hexagonal and tetragonal systems. On rotating the calcite rhomb one dot remained stationary but the other dot rotated with the calcite about the stationary dot. The ray corresponding to the image which moved is called the Extraordinary Ray - epsilon. The ray corresponding to the stationary image, which behaves as though it were in an isotropic mineral is called the Ordinary Ray - omega. The vibration direction of the ordinary ray lies in the {0001} plane of the calcite and is at right angles to the c-axis. The extraordinary ray vibrates perpendicular to the ordinary ray vibration direction in the plane which contains the c-axis of the calcite. If instead of using a calcite rhomb we had used a slab of calcite which had been cut in a random orientation and placed that on the dots, two images would still appear. If the random cuts were such that they were perpendicular to the c-axis, then light travelling through the calcite, along the c-axis would produce only one image andwould not become polarized. UNIAXIAL OPTIC SIGN In Calcite n omega > n epsilon, 1.658 versus 1.485. In other minerals, e.g. quartz, n omega < n epsilon , 1.544 versus 1.553. This difference in this refractive index relationship provides the basis for defining the optic sign of uniaxial minerals. Optically positive uniaxial minerals n omega < n epsilon (if extrordinary ray is the slow ray, then the mineral is optically positive.) Optically negative uniaxial minerals n omega > nepsilon (if extraordinary ray is the fast ray, then the mineral is optically negative.) UNIAXIAL INDICATRIX The indicatrix is a geometric figure, constructed so that the indices of refraction are plotted as radii that are parallel to the vibration direction of light. In isotropic minerals the indicatrix was a sphere, because the refractive index was the same in all directions. In uniaxial minerals, because n omega and n epsilon are not equal, the indicatrix is an ellipsoid, the shape of which is dependant on its orientation with respect to the optic axis. In positive uniaxial minerals, the Z indicatrix axis is parallel to the c-crystallographic axis and the indicatrix is a prolate ellipsoid, i.e. it is stretched out along the optic axis. For optically negative minerals the X indicatrix axis corresponds to the optic axis and the indicatrix is an oblate ellipsoid, i.e. flattened along the optic axis, and n omega > n epsilon In each case, for positive and negative minerals the circular section through the indicatrix is perpendicular to the optic axis and has a radius = nomega. The radius of the indicatrix along the optic axis is always nepsilon. Any section through the indicatrix which includes the optic axis is called a principal section, and produces an ellipse with axes nomega and nepsilon. BIREFRINGENCE AND INTERFERENCE COLOURS Birefringence, difference between the index of refraction of the slow and fast rays and the interference colours for uniaxial minerals is dependant on the direction that light travels through the mineral. 1- In a sample which has been cut perpendicular to the optic axis, the bottom and top surfaces will be parallel. The angle of incidence for the light entering the crystal = 0° and the wave front are not refracted at the interface and remain parallel to the mineral surface. A cut through the indicatrix, parallel to the bottom of the mineral, will yield the indices and vibration directions of the light. A slice through the indicatrix is a circular section, with radius nomega. No preferred vibration direction, so light passes along the optic axis as an ordinary ray and retains whatever vibration direction it had originally. Between crossed polars the light passing through the mineral is completely absorbed by the upper polar and will remain black on rotation of the stage, The birefringence = 0. 2- Cutting the sample such that the optic axis is parallel to the surface of the section the following is observed. The indicatrix section is a principle section, as it contains the optic axis. The indicatrix forms an ellipse with axes = n omega and n epsilon, with the incident light being split into two rays such that: the ordinary ray vibrates perpendicular to the optic axis, the extraordinary ray vibrates parallel to the optic axis. The birefringence is at a maximum, and in thin section this grain orientation will display the highest interference colour. 3- A mineral cut in a random orientation, with normally incident light; The ordinary ray produced has an index, nomega and vibrates perpendicular to the optic axis. The extraordinary ray has an index nepsilon' and vibrates in the plane containing the optic axis. nepsilon' < nomega maximum or minimum, the birefringence is intermediate between the two extremes. EXTINCTION IN UNIAXIAL MINERALS Uniaxial minerals will exhibit all four types of extinction discussed earlier. The type is dependent on: the orientation that the mineral is cut the presence of cleavage(s) in the grain PLEOCHROISM IN UNIAXIAL MINERALS Pleochroism is defined as the change in colour of a mineral, in plane light, on rotating the stage. It occurs when the wavelengths of the ordinary & extraordinary rays are absorbed differently on passing through a mineral, resulting in different wavelengths of light passing the mineral. If the colour change is quite distinct the pleochroism is said to be strong. If the colour change is minor = weak pleochroism. For coloured uniaxial minerals, sections cut perpendicular to the c axis will show a single colour, corresponding to ordinary ray. Sections parallel to the c crystallographic axis will exhibit the widest colour variation as both omega and epsilon are present. OBTAINING AN INTERFERENCE FIGURE To obtain and observe an interference figure using the microscope. With high power, focus on a mineral grain free of cracks and inclusions Flip in the auxiliary condensor and refocus open aperture diaphragm up to its maximum. Cross the polars Insert the Bertrand lens or remove the ocular and look down the microscope tube. Will not see the grain, but the interference figure, which appears on the top surface of the objective lense. The interference figure consists of a pattern of interference colours and a black band which may form a cross. Nature and pattern for the figure is dependent on the orientation of the grain. For Uniaxial Minerals three types of interference figures will be considered. Optic Axis Figure - OA vertical Off Centred Optic Axis Figure - OA inclined. Flash Figure - OA horizontal If the optic axis of the mineral is vertical, the grain will exhibit 0 birefringence and remain black or nearly black upon rotating the stage. OFF CENTRED OPTIC AXIS FIGURE The interference figure is produced when the optic axis is not vertical, resulting in the interference figure, i.e. the melatope, no longer being centred in the field of view. FLASH INTERFERANCE FIGURE A mineral grain is oriented with it's optic axis horizontal. This orientation exhibits the maximum birefringence, for this mineral in the thin section, and produces a flash figure. The flash figure results because the vibration directions, of the indicatrix, within the field of view are nearly parallel to polarisation directions of the microscope. extraordinary rays vibrate parallel to optic axis ordinary rays vibrate perpendicular to optic axis The c axis is parallel to stage. The isogyres split and leave field of view rapidly with only a slight rotation, <10°. The maximum interference colour will be observed under crossed polars. BIAXIAL MINERALS Include orthorhombic, monoclinic and triclinic systems, all exhibit less symmetry than uniaxial and isotropic minerals. It is also necessary to specify 3 different indices of refraction for biaxial minerals: N alpha, n beta, n gamma are used in text. where n alpha < n beta < n gamma The maximum birefringence of a biaxial mineral is defined by (ngamma - nalpha) It takes 3 indices of refraction to describe optical properties of biaxial minerals, however, light that enters biaxial minerals is broken into two rays - FAST and SLOW. The rays are both extraordinary and are referred to as SLOW RAY and FAST RAY. N slow = n gamma' , between n beta and n gamma (higher RI) N gamma > n gamma' > n beta N fast = n alpha' , between nalpha and n beta (lower RI) N alpha < n alpha' < n beta Thank You